Recently, people are more concerned about environmental problems and depletion of natural resources and, as such, interest in solar cells as an alternative energy source which does not cause environmental pollution is growing. Solar cells are classified into silicon solar cells, thin film-type compound solar cells, layered-type solar cells and the like. Among these solar cells, silicon semiconductor solar cells have been studied the most widely.
However, thin film type compound solar cells are recently studied and developed to improve problems of silicon solar cells.
Among thin film type compound semiconductors, Cu(In1-xGax)(SeyS1-y) (CI(G)S), which is a Group I-III-VI compound included in ternary compounds, has a direct transition type energy band gap of 1 eV or more and high light absorption coefficient. In addition, the Cu(In1-xGax)(SeyS1-y) (CI(G)S) is a stable electro-optically. Thus, the Cu(In1-xGax)(SeyS1-y) (CI(G)S) is a very ideal material as a light absorption layer of solar cells.
CI(G)S based solar cells are made of a thin layer having a thickness of several microns. Such a thin layer is manufactured using a variety of physicochemical thin layer manufacturing methods such as co-evaporation, sputtering, chemical bath deposition (CBD), selenization, spray pyrolysis and the like.
U.S. Pat. No. 4,523,051 discloses a method of manufacturing a highly efficient absorption layer using a co-evaporation method of Cu, In, Ga and Se under vacuum. However, when a large-area absorption layer is manufactured using the method, homogeneity is reduced. Although absorption layer formation through the vacuum process is advantageous to manufacture of highly efficient CI(G)S thin layer solar cells, initial investment of facilities and equipments such as sophisticated vacuum apparatus and the like is required and it is limited to lower price due to low material usage efficiency.
As another method, after forming Cu, In and Ga layers using a method such as sputtering, deposition or the like, a method of manufacturing a homogenous large-area absorption layer a large area absorption layer via selenization under a Se or H2Se atmosphere is known (See, Solar Energy 2004, Vol. 77, pages 749-756). However, such a method is not suitable for mass production due to long process time. In addition, a manufacturing process of the method is complex and thereby manufacturing costs are increased. Furthermore, the method uses hydrogen selenide which is a poisonous gas.
Recently, a CI(G)S thin layer formation method by thermal treating after coating a precursor material under non-vacuum was introduced. By using such a process, process costs may be reduced and a large area thin layer may be manufactured. However, the process has low absorption layer efficiency.
Namely, CI(G)Se thin layers formed by coating CI(G)Se nano particles under non-vacuum have a lot of gas pockets and are not dense. Thus, although densification of thin layers is generally performed via post heating, melting points of CI(G)Se materials are extremely high, 1000° C. or higher, and thereby particles growth and densification of even CI(G)Se compound nano particles of tens-of-nano sizes via post heating are not easy.
Therefore, there is a high need to develop a technology for thin film solar cells which is stable from oxidation and includes high efficiency light absorption layers formed using a fabrication method that is less expensive and safer than an existing CI(G)S thin film preparation method.